1. Field of the Invention
This invention relates to a programmable device that disables a ring signal sent by a subscriber line interface circuit (SLIC) of a programmable switch in a telecommunications system when a subscriber telephone is placed in an off-hook condition. The present invention also relates to methods for disabling such a ring signal.
2. Related Art
During a telephone call, it is very important to quickly cut (disable) the ring current after a subscriber answers an incoming call (i.e., places the telephone in an off-hook condition). If the ring current is not quickly disabled, then the high voltage ring current will flow through the subscriber's line and damage the subscriber's telephone unit.
The traditional methods of disabling the ring current when a subscriber places the telephone in an off-hook condition include hardware and software approaches. As described below, these methods are complicated, inflexible, and/or too slow to properly cut the ring signal current to the subscriber's telephone.
FIG. 1 is a block diagram of a conventional programmable switch 100 that uses a hardware approach for disabling the ring current when a subscriber (i.e., user) answers a telephone call. Programmable switch 100 includes a subscriber line interface circuit (SLIC) 101, hardware cut ring current (HCRC) device 102, ring relay 103, ring generating circuit 104, coder/decoder (CODEC) 105 and central processing unit (CPU) 106. CODEC 105 and CPU 106 are shown in dashed lines because these blocks are not used to cut the ring current in the hardware approach of FIG. 1.
SLIC 101, which is located at the subscriber line interface of programmable switch 100, is connected to the subscriber's telephone unit 111 by a telephone line 110. When a call is placed to the subscriber, ring generating circuit 104 generates a ring signal RING, which is a high voltage AC signal (e.g., 70–90 Volt AC). Ring relay 103 is closed, thereby routing the RING signal to SLIC 101 as the RING_OUT signal. SLIC 101 then routes the RING_OUT signal to the subscriber's telephone unit 111 on telephone line 110, thereby causing this telephone unit to ring. The RING_OUT signal results in a high ring current on telephone line 110. When the subscriber answers the call, telephone unit 111 is placed in an off-hook condition. SLIC 101 detects the off-hook condition on telephone line 110, and in response, activates an off-hook detected signal (DET) at an output terminal of SLIC 101.
The activated DET signal is transmitted from the output terminal of SLIC 101 to HCRC device 102. HCRC device 102 uses detailed hardware circuit signal processing in order to activate a ring cut signal (RING_CUT) in response to the activated DET signal. The activated RING_CUT signal is applied to ring relay 103, thereby causing ring relay 103 to switch to an open state. When in the open state, ring relay 103 disconnects the signal path between ring generating circuit 104 and SLIC 101. As a result, the RING signal is no longer routed through ring relay 103 as the RING_OUT signal and the ring current is unable to reach SLIC 101 or the subscriber's telephone. Cutting off the ring current in this manner protects the subscriber's telephone from damage.
The advantage of the hardware cut ring method is the fast speed of the ring cut process. However, HCRC device 102 requires a complicated control circuit, which is assembled using discrete devices. Moreover, after HCRC device 102 of FIG. 1 has been designed, the high and low voltage levels of the input and output signals (DET and RING_CUT) are fixed, and are therefore not programmable.
FIG. 2 is a block diagram of a conventional programmable switch 200 that uses a software approach for cutting the ring current when a subscriber answers a telephone call. Programmable switch 200 includes a subscriber line interface circuit (SLIC) 201, ring relay 203, ring generating circuit 204, coder/decoder (CODEC) 205 and central processing unit (CPU) 206. Programmable switch 200 utilizes CPU 206 for software control, in lieu of using hardware cut ring current device 102 of FIG. 1.
SLIC 201, ring relay 203 and ring generating circuit 204 operate in a manner similar to SLIC 101, ring relay 103 and ring generating circuit 104 of programmable switch 100 (FIG. 1). Thus, when a call is placed to the subscriber, ring generating circuit 204 generates the RING signal, which is routed through ring relay 203 and SLIC 201 to the subscriber's telephone unit 211 on telephone line 210, thereby causing this telephone unit to ring. When the subscriber answers the call, the telephone is placed in an off-hook condition. SLIC 201 detects the off-hook condition on telephone line 210, and in response, activates an off-hook detected signal (DET) at an output terminal of SLIC 201.
The activated DET signal is transmitted from the output terminal of SLIC 201 to CPU 206. CPU 206 processes the DET signal with programmed software. When the software determines that the DET signal has been properly activated, CPU activates a ring cut signal (RING_CUT) in response to the activated DET signal. The activated RING_CUT signal is applied to ring relay 203, thereby causing ring relay 203 to switch to an open state. When in the open state, ring relay 203 disconnects the signal path between ring generating circuit 204 and SLIC 201. As a result, the ring current is cut off from SLIC 201 and the subscriber's telephone unit 211. Again, cutting off the ring current in this manner protects the subscriber's telephone from damage.
The advantages of the software cut ring method implemented by programmable switch 200 include the simplicity of the circuit, the flexibility of the circuit, and the capability of re-programming the circuit. One disadvantage of the software cut ring method is that CPU 206 requires a relatively long processing time before activating the RING_CUT signal. This relatively long processing time can result in a significant delay in cutting off the ring current, thereby resulting in damage to subscriber's telephone unit 211. Another disadvantage of the software cut ring method is that CPU 206 may fail to activate the RING_CUT signal during a software testing period or during a period in which CPU 206 is malfunctioning. In these cases, CPU 206 will fail to cut off the ring current, thereby resulting in damage to the subscriber's telephone unit 211.
It would therefore be desirable to have a method and apparatus for cutting off ring current, which overcomes the above-described deficiencies of the hardware and software cut ring current methods. That is, it would be desirable to have a method and apparatus for cutting off ring current that is capable of quickly cutting off the ring current upon detecting an off-hook condition, while having the advantage of being programmable.